The invention relates to an ion-implantation apparatus of a sheet type, and more particularly to an ion-beam scanning control feature involved in an ion-implantation apparatus of a parallel scanning type which accomplishes an electrostatic X-scanning of an ion-beam and mechanical Y-scanning.
In general, the sheet type of ion-implantation apparatus has been used for ion-implantation in a medium current region. Such sheet type ion-implantation apparatus may be divided into the following two types. One is an electrostatic scanning type of ion-implantation apparatus which accomplishes X-electrostatic scanning and Y-electrostatic scanning of ion-beams at predetermined frequencies respectively. Another is a parallel scanning type ion-implantation apparatus which accomplishes X-electrostatic scanning of the ion-beam and mechanical Y-scanning at predetermined frequencies respectively.
In the prior art, such a parallel scanning type of ion-implantation apparatus has the following structures. With reference to FIG. 1, the conventional structure of the ion-beam scanning control feature involved in the parallel scanning type ion-implantation apparatus has a platen 2 on which a wafer 1 is provided. The platen 2 is supported by a Y-scanning feature 3 so as to be movable in the vertical direction, or Y-direction thereby accomplishing the mechanical Y-scanning in the vertical direction.
In contrast to such a mechanical scanning in the Y-direction, the ion-beam 5 scans in the X-direction at a predetermined frequency (typically about 500 Hz) in the horizontal direction, controlled by an electrostatic scanning system which employs a pair of X-deflector electrode plates 4. Further, such a conventional structure of the ion-beam scanning control includes two Faraday tubes 6 serving as detecting elements, which are arranged at diametrically opposite sides approximately and behind the platen 2. The Faraday tubes 6 are supported to be in separation from the platen 2 which is movable in the vertical direction. Faraday tubes 6 catch the ion-beam and makes a measurement of a current generated by the ion-beam 5.
The operation of the ion-beam scanning control feature involved in the parallel scanning type ion-implantation apparatus will be described with reference to FIG. 1. The ion-beam 5 is transmitted to the wafer 1 on the platen 2 via a space defined by a pair of the X-deflector electrode plates 4. The ions having a predetermined average energy supplied by the ion-beam 5 are implanted into the wafer 1. The X-scanning of the ion-beam 5 has a predetermined frequency which is controlled by voltage signals applied to the X-deflector electrode plates 4. The Y-scanning feature 3 moves the platen 2 on which the wafer 1 is supported in the Y-direction, or vertical direction thereby accomplishing the mechanical Y-scanning.
The problem with such X-scanning by the ion-beam 5 is an over-scanning which provides unnecessary ion-beams to which external area and thus an opposite areas beyond the surface of the wafer 1 is subjected. To prevent such over-scanning, the conventional apparatus includes a feature for detecting the over scanning. In the X-scanning controlled by a pair of the X-deflector electrode plates 4, if there is any over-scanning of the ion-beam 5, which has a larger scanning width than the diameter of the wafer 1, the Faraday tubes 6 catches the over-scanning ion-beams. Hence, the Faraday tubes 6 detect the over-scanning. When the over-scanning of the ion-beam 5 is detected by the Faraday tubes 6, voltage signals are applied to a pair of the X-deflector electrode plates 4 which provide a predetermined scanning frequency. The scanning frequency provided by such X-deflector electrode plates 4 is determinative of the scanning width of the ion-beam 5 in the X-direction, or the horizontal direction so as to limit the scanning width of the ion-beam 5 to the diameter of the wafer 1 supported on the platen 2. As a result, such scanning ion-beam 5 has an uniformity in the X-scanning width which is defined by the distance between the two Faraday tubes 6.
Although the wafer 1 is circular, such a scanning area of the ion-beam 5 is defined into a square to internally touch the circle defined by the wafer 1. Since the wafer 1 which is subjected to the ion-implantation is circular, such conventional ion-beam scanning having the uniformity in the X-scanning width and thus having a square ion-implantation area is forced to have unnecessary over-scanning of such square area. Then, four corner areas in the square ion-implantation area except for an overlapping area to the wafer is subjected to the over-scanning of the ion-beam. It is, therefore, necessary to develop a novel scanning control feature which is free from any over-scanning, or unnecessary scanning. Further, such over-scanning to the external area to the wafer requires an unnecessary time for an ion-implantation process. It is, thus, desirable to prevent any over-scanning.